Metal alloys having dendritic crystal structure at ambient temperature conventionally have been melted and then subjected to high pressure die casting procedures. Such conventional die casting procedures have certain problems associated therewith such as melt loss, contamination with flux or the like, excessive scrap, rather high energy consumption, somewhat lengthy duty cycles, limited die life due to high thermal shock or the like, and restricted die filling positions. The alloys involved include, but are not limited to, alloys described in U.S. Pat. Nos. 3,840,365; 3,842,895; 3,902,544; and 3,936,298.
Plastics injection molding techniques have many features which would be advantageous if they could be included in the injection molding of such metal alloys which can be converted into a thixotropic state. Such techniques include the feeding of plastic granules at room temperature from a hopper into a screw extruder in the absence of flux and other impurities. The plastic material is heated in the extruder to become plasticized, following which a mold positioned at the discharge end of the extruder is filled with the flowable material. There are no contamination and melt losses associated with plastic extrusion procedures, and the lower temperatures utilized in such procedures reduce the problem of thermal shock to the mold. In injection molding of plastics, the mold can be filled from any position as dictated by maximum efficiency for part fillings. Apparatus and methods according to the invention include most, if not all, of these desirable characters.
U.S. Pat. Nos. 4,694,881 and 4,694,882 disclose the conversion of a metal alloy having dendritic properties into a thixotropic, semi-solid state by controlled heating so as to maintain the alloy at a temperature above its solidus temperature and below its liquidus temperature while subjecting the alloy to a shearing action during injection molding. In this manner certain advantages of injection molding can be utilized to overcome certain disadvantages of die casting. The present invention incorporates additional improvements and advantages resulting from the injection molding of metal alloys.